1 Introduction. Research article

Transcription

1 Nanophotonics 2018; 7(4): Research article Huifu Xiao, Dezhao Li, Zilong Liu, Xu Han, Wenping Chen, Ting Zhao, Yonghui Tian* and Jianhong Yang* Experimental realization of a CMOS-compatible optical directed priority encoder using cascaded micro-ring resonators Received January 10, 2018; revised February 1, 2018; accepted February 1, 2018 Abstract: In this paper, we propose and experimentally demonstrate an integrated optical device that can implement the logical function of priority encoding from a 4-bit electrical signal to a 2-bit optical signal For the proof of concept, the thermo-optic modulation scheme is adopted to tune each micro-ring resonator (MRR) A monochromatic light with the working wavelength is coupled into the input port of the device through a lensed fiber, and the four input electrical logic signals regarded as pending encode signals are applied to the micro-heaters above four MRRs to control the working states of the optical switches The encoding results are directed to the output ports in the form of light At last, the logical function of priority encoding with an operation speed of 10 Kbps is demonstrated successfully Keywords: optical logic devices; integrated optics devices; optical processing devices; photonic integrated circuits 1 Introduction With the development of supercomputing and optical information processing, researching on optical logic *Corresponding authors: Yonghui Tian and Jianhong Yang, Institute of Microelectronics and Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou , China, siphoton@lzueducn (Y Tian), yangjh@lzueducn (J Yang) (Y Tian) Huifu Xiao, Dezhao Li, Zilong Liu, Xu Han, Wenping Chen and Ting Zhao: Institute of Microelectronics and Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou , China schemes is becoming increasingly crucial [1] Among all the optical logic schemes, directed logic is a novel and promising one which employs the optical switch network to perform logical operation Each optical switch in directed logic circuit is independent of each other, which means the latency of each optical switch cannot be accumulated and the total latency time is very small compared to those in electrical logic domain [2] For directed logical scheme, monochromatic light with specific working wavelength is coupled into the optical switch network, and electrical Boolean signals regarded as logical operands are applied to the optical switches in optical network to control their working states Then the final logical operation results are directed to the output ports of the optical switch network in the form of light, which means the control signal for the directed logic is electrical signal and the operation signal is light signal [1, 2] As we know, the electrical signal is easy to control owing to the existing mature techniques, and the optical signal is a very prospective solution for logic operation due to its nature features such as large bandwidth, low latency, and parallel processing In other words, the directed logic combines the advantages of both electron and photon but avoids their drawbacks Therefore, directed logic is a promising information processing scheme, which is well suitable for various application fields such as packet routing, digital optical logic, and real-time applications [1 3] Basically, directed logic circuits comprise on-chip integrated optical switches There are two typical different kinds of optical switches for integrated on-chip schemes The first one is Mach-Zehnder-based optical switch, which usually has large size, high-power consumption, etc Therefore, this kind of optical switch is not convenient for large-scale integration The other one is micro-ring resonator (MRR)-based optical switch fabricated on siliconon-insulator (SOI) wafer, which is a fundamental building block for the application of on-chip large-scale integration owing to its outstanding properties such as compact size, low power consumption, and complementary metal Open Access 2018 Yonghui Tian and Jianhong Yang et al, published by De Gruyter Attribution-NonCommercial-NoDerivatives 40 License This work is licensed under the Creative Commons

2 728 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder oxide semiconductor (CMOS) compatible process Currently, various MRR-based silicon photonic devices have been proposed and demonstrated such as modulator [4, 5], filter [6, 7], optical logic device [8 10], mathematical solver [11], and sensor [12] All these demonstrated photonic devices have provided significant application prospects for optical information processing Based on above discussions, MRR-based optical switch is a very suitable structure for us to build the optical network to achieve optical directed logic As we know, optical logic devices are fundamental elements for optical information processing system since all information processing can be attributed to logic operations Just like other logic devices, optical encoder is a key building block for optical information processing, by which the multiple binary inputs can be converted into several outputs with specific meanings [13 15] In our previous work, a general optical directed encoder which can encode a 4-bit electrical signal to a 2-bit optical signal has been demonstrated successfully [14]; however, this kind of optical encoder does not have the function of priority which is very important for optical information processing The priority encoder, which can identify the priority levels of the request signals and encode them with specific meanings, is indispensable for applications in on-chip optical network, control system, emergency responders, etc Although a theoretical realization of priority encoder has been proposed using MRRs [15], the structure of the device is relatively complicated, and there is no experimental demonstration In this paper, we propose and experimentally demonstrate a CMOScompatible photonic device that can achieve the function of optical priority encoding To our best knowledge, it is the first actualization of integrated optical priority encoder fabricated on SOI substrate The proposed priority encoder can identify the priority level of an arbitrary 4-bit electrical signal and encode it to a specific 2-bit optical signal 2 Device principle The schematic of the proposed device composed of four MRRs and four normal waveguides is shown in Figure 1A Micro-heaters fabricated several micrometers above the MRRs are employed to modulate MRRs basing on thermooptic effect (Figure 1B) [16, 17] To elaborate the function of the priority encoder, ports of the device are defined as Input, Y 1,, IDLE, and X The port IDLE is regarded as the test terminal which can identify whether the device is in working state For example, if there is no light signal Figure 1: The device is designed and fabricated The proposed device is designed and fabricated which consists of four cascaded micro-ring resonators, (A) is the schematic of the proposed device, the corresponding ports are defined Input, Y 1, IDLE,, and X; (B) is the micrograph of the optical priority encoder (I 1 ~ : input electrical pulse sequences, MRR: micro-ring resonator) (logic 0) at the IDLE port, the device is in encoding state, and the converse is not Monochromatic continuous wave at the working wavelength of λ ω is launched into the port Input and modulated by the electrical signals applied to the micro-heaters on MRR 4, respectively The four MRRs are initially designed to have the same parameters, and thus they theoretically have the same resonant wavelengths λ 0 Here, we define that MRR is in ON state at the working wavelength of λ ω (λ ω > λ 0 ) when the electrical input signal applied to the corresponding MRR is at high level (logic 1), in which situation the light will be downloaded to the drop port of the corresponding MRR When the electrical input signal applied to the corresponding MRR is at low level (logic 0), the light would pass by the MRR and be directed to the through port, in which state the MRR is defined as OFF state The input electrical signal (a 4-bit electrical signal I 3 I 1 ) applied to the MRRs can be encoded to a 2-bit optical signal according to their priority level, and the priority encoding results can be obtained at the output ports Y 1 in the form of light Similarly, logic 1 and 0 are employed to define the high and low levels of optical power in optical domain In one case, if all the four MRRs are OFF at the input working wavelength λ ω, the launched monochromatic light wave with the working wavelength λ ω would be directed to the port IDLE, which indicates that the output power at port IDLE is at high level (logic 1), and the function of priority encoding is not working In other cases, if one or more MRRs are ON at the input working wavelength

3 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder 729 λ ω, the launched light wave would be directed to the drop ports of corresponding MRRs and the function of priority encoding would take effect; meanwhile, the output power at port IDLE would be at low level (logic 0) The combinations of the output logic at ports Y 1 act as the encoding results of the proposed optical priority encoder Therefore, only if the output optical power of port IDLE is at low level (IDLE = 0) will the function of priority encoding be effective The detailed working principle of the proposed optical priority encoder is elaborated as follows: when is ON ( = 1), the light will resonate in MRR 4 and be directed to port X No matter what the other input logics (I 3,, I 1 ) are, the optical power will be at low levels in both ports Y 1 (Y 1 = 0) In this case, has the highest priority, and any logical inputs of I 3,, will obtain the same results since they are taken precedence over by the highest priority of the input Similarly, when I 3 = 1, the light will pass by MRR 4 firstly and then resonate in MRR 3 before being directed to port The optical power will be at high level in port and at low level in Y 1 port in this working state (Y 1 = 1), and any input logics of will make no sense to the output result When I 3 and = 1, the light will firstly pass by MRR 4 and MRR 3 before resonating in MRR 2, and the optical power will be at high level at Y 1 port while at low level at port (Y 1 = 1, = 0) When I 3 I 1 = 1, the light will be directed to both Y 1 since MRR 1 works as a power splitter The optical powers will be at high levels at both Y 1 ports (Y 1 = 1, = 1) In a word, the proposed device can perform the priority encoding function from a 4-bit electrical signal to a 2-bit optical signal, and the priority levels of the input signals can be described as > I 3 > > I 1 The logic equations for the output results of priority encoder are described as follows: IDLE = IIII Y = I II + IIII Y = I I + I III According to the working principle described above, the truth table of the priority encoder shown in Table 1 can Table 1: Truth table of the optical priority encoder I 3 I 1 IDLE Y 1 1 x x x x x x (1) be obtained, where x indicates an arbitrary logic signal (logic 1 or 0) 3 Fabrication and experimental The device is fabricated on an eight-inch SOI wafer with 220 nm-thick top silicon layer and 2 μm-thick buried SiO 2 layer (Figure 1B) in Institute of Microelectronics, Singapore The rib waveguides with 400 nm in width, 220 nm in height, and 90 nm in slab thickness are employed to form the device, which only support quasi-transverse electric fundamental mode The radii of MRRs are 10 μm, and the coupling gaps between straight waveguides and ring waveguides are 350 nm The top cladding oxide with the thickness of 2 μm is deposited on the top of MRR by plasma enhanced chemical vapor deposition, and then four TiN micro-heaters with widths of 2 μm and thicknesses of 120 nm are fabricated to modulate the MRRs Aluminum trances are formed to connect the micro-heaters and the μm 2 pads at last In order to improve the coupling efficiency between the external lensed fiber and micro-waveguide, spot size converters are integrated on the termination ports of the device In order to determine the working wavelength and analog voltages of each electrical input signal, an amplified spontaneous emission (ASE) source, two 2-channel tunable voltage sources (TVSs), and an optical spectrum analyzer (OSA) are employed to characterize the static response spectra of the device Broadband light generated by the ASE is coupled into the input port of the device through a lensed fiber, and the output light is fed into the OSA through another lensed fiber Four electrical input signals generated by the TVSs are used to drive the microheaters above the MRRs When the MRR is heated up, the refractive index of the silicon increases basing on thermooptic effect, and the resonant wavelength of the MRR will red-shift Although all MRRs are designed to have the same structural parameters, they have slightly different resonant wavelengths due to the limited manufacturing accuracy These differences can be offset by applying different voltages to different MRR In order to achieve a sufficiently large extinction ratio and a low power consumption, the working wavelength λ ω is chosen to be nm The resonant wavelengths of MRR 4 can be tuned to nm when the amplitudes of applied voltages are 154 V, 169 V, 178 V, and 146 V, respectively, which means the applied voltages of, I 3,, for being ON are 154 V, 169 V, 178 V, and 146 V, respectively There are 16 kinds of logic combinations for four electrical

4 730 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder input signals, and we measured all the 16 static response spectra of the device at output ports Y 1 as shown in Figures 2 and 3, respectively From Figure 2A, we can see that the output power is at low level ( 3339 db) at nm when no electric signals are applied on MRRs, which means when = I 3 = = I 1 Y 1 = 0 The optical power at the output of Y 1 is at high level ( 1509 db) at nm when only I 1 is ON (Figure 2B), which means when = I 3 = = 0 = 1, Y 1 = 1 When is ON, the output power is all at high levels ( 1097 db and 1103 db) at nm (Figure 2C and D), which means when = I 3 = 0 and = 1, no matter whether I 1 is 1 or 0, the output result in Y 1 would always be Y 1 = 1 When I 3 is ON and is OFF, the output power is all at low levels ( 4839 db, 3475 db, 2697 db, and 2694 db) at nm (Figure 2E H), which means that when = 0 and I 3 = 1, no matter what the logic combination of the is, the output of Y 1 would obtain Y 1 = 0 When is ON, the output power is all at low levels ( 4672 db, 3370 db, 2818 db, 2815 db, 4856 db, 4092 db, 3247 db and 3393 db) at nm (Figure 2I P), which means that when = 1, no matter what the logics of the I 3,, are, the output of Y 1 would always obtain Y 1 = 0 Note that in defining logics 1 and 0 of the output results, threshold power of 1650 db and 2650 db are adopted to distinguish the high and low levels The 16 static response spectra of the device at the output ports is shown in Figure 3 By the same way mentioned above, we can obtain the output results of port at the wavelength of nm In combination with the results of Figures 2 and 3, the results of the proposed priority encoder at the wavelength of nm are listed in Table 2 Comparing result no 2 with results no 3, 4, 5 8, and 9 16, we know that the proposed device can implement the encoding function from a 4-bit electrical signal to a 2-bit optical signal Comparing the logical result no 3 with no 4, we know that the priority level of is higher than I 1 since when = 1, no matter what the input logic of I 1 is, the encoded logical results will be the same Comparing the logical results no 5 8 against each Figure 2: Response spectra in the Y 1 port under different combinations of voltages In (A P), the logic 1 (0) of each signal represents that the corresponding voltage is at high (low) level The high levels of voltage applied to the micro-heaters above MRR 4 are 154, 169, 178, and 146 V, respectively, while the low levels are all 0 V

5 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder 731 Figure 3: Response spectra in the port under different combinations of voltages In (A P), the logic 1 (0) of each signal represents that the corresponding voltage is at high (low) level The high levels of voltage applied to the micro-heaters above MRR 4 are 154, 169, 178, and 146 V, respectively, while the low levels are all 0 V Table 2: Priority encoding results of the proposed device No Voltages applied to the corresponding MRRs Measured output results Logical results (V) I 3 (V) (V) I 1 (V) Y 1 (db) (db) (Y 1 )

6 732 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder other, we can know that the priority level of I 3 is higher than since when I 3 = 1, no matter what the input logics of are, the encoded logical results will be the same Comparing the combination results No 9 16, we can know that the priority level of is higher than I 3,,, since when = 1, no matter what the input logics of I 3,, are, the encoded logical results will be the same In a word, the static response spectra of Y 1 ports are in agreement with the analytical results and the priority levels of the four input signals are > I 3 > > I 1 The dynamic performance of the device is further demonstrated According to the static response spectra of the device, the input working wavelength is chosen to be nm, and the high levels of voltages applied on MRR 4 are 154 V, 169 V, 178 V, and 146 V, respectively, while the low levels are all 0 V Monochromatic light generated by tunable laser with the wavelength of nm is coupled into the Input port of the device Four continuous binary sequences generated by four arbitrary function generators with specific high and low levels of voltages at 10 kbps are applied to the four MRRs, respectively The applied sequences are with periods of 16 bits, and each bit is 01 ms in length Although the working speed of 10 kbps is relatively slow compared with the speed of electro-optic modulation schemes, it is enough for the proof of concept of the optical priority encoder since the fabrication process of thermo-optic modulation scheme is much simpler and the cost is much lower The light obtained at the output ports Y 1 are fed into a photo-detector (PD) to transform the light signal to electrical signal The four electrical signals applied to four MRRs and the electrical signals transformed by the PD are observed by an oscilloscope The measured dynamic response results are shown in Figure 4 As we can see, if is ON ( = 1), the optical power at output ports Y 1 are both at low levels (Y 1 = 0) no matter what the other electrical input signals (I 3,, I 1 ) are, since has the highest priority Similarly, when = 0 and I 3 = 1, no matter what the other electrical input signals (, I 1 ) are, the encoding result is Y 1 = 1 since the priority of I 3 is higher than those of When I 3 and = 1, the encoding result is Y 1 = 1, regardless of whether I 1 is ON or OFF since the priority of is higher than I 1 When and only when I 3 = 0 = 1 will the encoding result be Y 1 = 1, = 1 From the dynamic results, we can conclude that the device can perform the priority encoding function from a 4-bit electrical signal to a 2-bit optical signal correctly, and the priority levels of four input signals are > I 3 > > I 1 The maximum rising and falling times are about 17 μs and 28 μs for the obtained results at Y 1 port, while these are 31 μs and Figure 4: The dynamic performances of the device are measured (A D) are signals applied to MRR 4, respectively (E) is the result at the output port Y 1, and (F) is the result at the output port 20 μs for the obtained results at port, respectively Note that the power levels for logic 1 are different at both the output ports Y 1 in different cases, which is mainly the result from the three coupling regions of MRR 1 MRR 1 behaves as a power splitter in this device, half of the resonant light is directed to the output port Y 1 and the other half of the resonant light is directed to the output port Therefore, the power levels for logic 1 have steps Similar phenomena can also be observed in the static response spectra However, this does not hinder the implementation of the priority encoding function of the device 4 Conclusion In conclusion, for proof of concept, we have proposed and experimentally demonstrated a directed logic device

7 H Xiao et al: Experimental realization of a CMOS-compatible optical directed priority encoder 733 which can implement the priority encoding function from a 4-bit electrical signal to a 2-bit optical signal The static response spectra and dynamic results of the device are successfully demonstrated To achieve higher speed of operation, we can employ other advanced modulation schemes, such as the plasma dispersion effect [18], to modulate the MRRs Acknowledgments: This work was supported in part by the National Natural Science Foundation of China under Grant and by the Fundamental Research Funds for the Central Universities under Grant lzujbky-2016-k05, the Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education (LZUMMM ), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2016KF14) References [1] Caulfield HJ, Dolev S Why future supercomputing requires optics Nat Photon 2010;4:261 3 [2] Hardy J, Shamir J Optics inspired logic architecture Opt Express 2007;15: [3] Ramaswami R, Sivarajan KN Design of logical topologies for wavelength-routed optical networks IEEE J Sel Areas Commun 1996;14: [4] Dong P, Shafiiha R, Liao S, et al Wavelength-tunable silicon microring modulator Opt Express 2010;18: [5] Xu QF, Manipatruni S, Schmidt B, Shakya J, Lipson M 125 Gbit/s Carrier-injection-based silicon micro-ring silicon modulators Opt Express 2007;15:430 6 [6] Little BE, Chu ST, Haus HA, Foresi J, Laine J-P Microring resonator channel dropping filters J Lightwave Technol 1997;15: [7] Dong JJ, Liu L, Gao DS, et al Compact notch microwave photonic filters using on-chip integrated microring resonators IEEE Photonics J 2013;5: [8] Qiu CY, Ye X, Soref R, Yang L, Xu QF Demonstration of reconfigurable electro-optical logic with silicon photonic integrated circuits Opt Lett 2012;37: [9] Xu QF, Lipson M All-optical logic based on silicon micro-ring resonators Opt Express 2007;15:924 9 [10] Zhang L, Ji RQ, Jia L, et al Demonstration of directed XOR/ XNOR logic gates using two cascaded microring resonators Opt Lett 2010;35: [11] Wu J, Cao P, Hu X, et al Compact tunable silicon photonic differential-equation solver for general linear time-invariant systems Opt Express 2014;22: [12] Yi H, Citrin D, Chen Y, Zhou Z Dual micro-ring resonator interference sensor Appl Phys Lett 2009;95: [13] Tian YH, Zhang L, Ji RQ, et al Demonstration of a directed optical decoder using two cascaded microring resonators Opt Lett 2011;36: [14] Tian YH, Zhang L, Ji RQ, Yang L, Xu QF Demonstration of a directed optical encoder using microring-resonator-based optical switches Opt Lett 2011;36: [15] Xu QF, Soref R Reconfigurable optical directed-logic circuits using microresonator-based optical switches Opt Express 2011;19: [16] Cocorullo G, Corte FGD, Rendina I Temperature dependence of the thermo-optic coefficient in crystalline silicon between room temperature and 550 K at the wavelength of 1523 nm Appl Phys Lett 1999;74: [17] Padmaraju K, Bergman K Resolving the thermal challenges for silicon microring resonator devices Nanophotonics 2013;3: [18] Chen QS, Zhang FF, Zhang L, et al 1 Gbps Directed optical decoder based on two cascaded microring resonators Opt Lett 2014;39:4255 8